The other problem is that if you have a motherboard failure you are forced to buy another board with the same chipset to be able to save your array unlike having a dedicated RAID card that would let you move it from PC to PC effortlessly and safely.

Requiring an identical controller to recover an array due to controller failure is not unique to motherboard RAID. If you are using a dedicated RAID card and your data is that important, I recommend you not only have spare drives on hand but a spare controller card as well.

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WHS also doesn't have "drives" like a regular OS, it sees all the HDDs as one big drive so you can keep adding drives and it just increases the total storage on the machine.

This is known as JBOD mode (Just a Bunch Of Disks) where they all get spanned into one large drive. While it can be useful in some circumstances it can also be a very dangerous way to store data and is not really recommended for anything critical.